Western military sources note a surprising improvement in the Syrian air force’s performance due to the help of Russian advisers in stabilizing operational and technical systems. In Hama alone, from early March up until May 20, Syria made 1,864 air sorties. Helicopter raids for dropping Iranian-made barrel bombs (each barrel is filled with small cluster bombs) represented 58 percent of the total. The remaining 42 percent were conducted by fixed-wing fighter jets which pounded targets with chlorine-filled bombs. Most of the additional bombardments took place over urban districts, whereas attacks on outlying regions in the south and north declined.